1. Field of the Invention
The invention relates to method for measuring the photoacoustic signal with computer-assisted evaluation.
2. Discussion of Background Information
Photoacoustic spectroscopy is becoming increasingly used as a measuring method to determine the presence or concentration of substances that absorb light. Excitation by means of light is used that is absorbed by the substances to be investigated. The absorption generates heat which causes the surrounding gas, liquid or solid body to expand. A pressure wave is generated by this heat. When periodic or pulsed excitation is used, it can generate a sound wave. This sound wave can be measured by means of a sound pressure sensor. The signal obtained thereby is a measure of the concentration of the substances to be investigated. A widespread measurement option is excitation by means of a modulated light source. When the excitation light is modulated, the corresponding acoustic signal must also be modulated. This insight is used to suppress the background signal. It is conventional to use lock-in amplifiers which use complicated electronics to filter out of the obtained signal those components that correspond to the modulation frequency. In this manner it is also possible to investigate several substances one after the other using different light sources that emit light at different wavelengths and are modulated with the modulation frequency. It is also possible to measure two substances simultaneously using two light sources that are modulated with the same modulation frequency but are modulated in antiphase. The disadvantage here is that the required measuring technique is very complicated and hence very expensive.
A set-up is for example known from U.S. Pat. No. 6,608,683 B1 in which a modulation frequency is provided by means of which the optical radiation entering the resonant photoacoustic measuring cell is modulated. The modulation frequency is simultaneously transmitted to a lock-in amplifier. The signal originating from a microphone is also transmitted to the lock-in amplifier. There, those signal components that correspond to the modulation frequency are amplified. The modulation frequency is adjusted depending on the determined photoacoustic signal.
U.S. Pat. No. 5,129,255 discloses a complex electronic circuit with which the signal obtained is amplified by resonant excitation of a photoacoustic measuring cell.
A photoacoustic measuring arrangement is known from EP 1 715 324 A1 for detecting gases and/or aerosols. This arrangement has a measuring cell and reference cell. The difference signal from the microphone of the measuring cell and the microphone of the reference cell is transmitted to a difference amplifier from where it is sent to a phase-sensitive rectifier, i.e. a lock-in amplifier.
A photoacoustic measuring arrangement is known from U.S. Pat. No. 4,200,399 in which the radiation originating from a radiation source is modulated by means of a known reference frequency. The modulated radiation is used to excite sound waves in a measuring cell by means of absorption, which sound waves are detected by a microphone. The microphone signal is transmitted to a lock-in amplifier. In the lock-in amplifier, those signals are amplified that correspond to a reference frequency.